1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
It is known to illuminate different parts of a pattern to be produced on a substrate with different illumination settings. For example, a desired illumination for a pattern consisting of elongate, relatively thin, features is so-called dipole illumination. In this arrangement, illuminating radiation is arranged in a pupil plane, such that the radiation is confined to portions of the pupil plane that are set apart from an optical axis along a line in the pupil plane that is parallel to the elongate features of the pattern to be illuminated.
However, patterns to be illuminated typically include not only elongate features parallel to a first direction, but also elongate features perpendicular to this direction. In this situation, typically the pattern is divided into two exposures: (1) the first exposure illuminating a sub-pattern including the elongate features parallel to a first direction and illuminated using the appropriately orientated dipole illumination and (2) the second exposure illuminating the perpendicularly orientated elongate features using dipole illumination orientated appropriately for those features. The disadvantage of such an arrangement is that two exposures are required for each complete pattern. Therefore, the exposure time is doubled, reducing the throughput of the apparatus.
A compromise solution has also been used, in which the complete pattern is illuminated using quadrupole illumination. This is illumination in which the radiation in the pupil plane is arranged as a combination of two dipole exposures, each optimized for illuminating patterns of elongate features perpendicular to one another. This arrangement avoids the loss of throughput for the apparatus, but the quality of the image projected onto the substrate is not as good as can be achieved with separate dipole illumination exposures. Additionally, it is desirable to use increasingly high NA (Numerical Aperture) when imaging patterns onto the substrate because this can further increase the quality of the pattern imaged onto the substrate. However, as the NA increases a grazing angle of the radiation projected onto the substrate also increases. Consequently, as the NA increases, the proportion of the radiation that is reflected from the substrate also increases. The reflected radiation is stray radiation that can affect other parts of the apparatus or reduce the quality of the pattern produced on the substrate if reflected back to the substrate. Furthermore, as the proportion of stray radiation increases, it is necessary to increase the intensity of the illumination radiation and/or increase the exposure time, thereby increasing the cost of the apparatus and/or reducing the throughput of the apparatus.
Therefore, what is needed is an improved arrangement for illuminating patterns and projecting them onto the substrate.